Silicon carbide (SiC), for example, has the outstanding properties of having a band gap roughly three times wider, dielectric breakdown electric field strength roughly ten times stronger, and thermal conductivity roughly three times greater than silicon (Si), and is expected to be used in applications such as power devices, high-frequency devices or high-temperature operation devices.
SiC epitaxial wafers are normally used to manufacture such SiC semiconductor devices. SiC epitaxial wafers are fabricated by epitaxially growing an SiC single crystal thin film (epitaxial layer) to serve as the active region of the SiC semiconductor device on the surface of an SiC single crystal substrate (wafer) fabricated using a method such as sublimation recrystallization.
In addition, a chemical vapor deposition (CVD) device, which deposits and grows SiC epitaxial layers on the surfaces of heated SiC wafers while supplying a raw material gas to a chamber, is used for the epitaxial wafer manufacturing device.
In this CVD device, the SiC wafers are required to be heated to a high temperature in order to induce epitaxial growth of the SiC epitaxial layers. Consequently, a method is used in which the susceptor on which the wafers are mounted and the ceiling (top plate) arranged opposing the upper surface of this susceptor are heated by high-frequency induction heating, and the wafers are heated by radiant heat from the susceptor and ceiling (see Patent Documents 1 and 2). Thus, susceptors and ceilings made of graphite (carbon) are used since they are suitable for high-frequency induction heating.
However, in a CVD device, deposits of the SiC epitaxial layers are deposited not only on the surfaces of the SiC wafers, but also on the surface of the ceiling during film formation. As a result of repeating film formation, there were cases in which deposits deposited on the surface of the ceiling separated from the ceiling and fell onto the surfaces of the SiC wafers.
In this case, the film quality of the SiC epitaxial layers was significantly impaired due to the deposits (particles) adhered to the surfaces of the SiC epitaxial layers and the deposits (downfall) embedded in the SiC epitaxial layers.
This type of problem is particularly prominent in volume production-type CVD devices that repeatedly carry out film formation. Consequently, CVD devices require that cleaning work be periodically performed to remove deposits deposited on the ceiling and other locations inside the chamber.
However, since the size of the chamber is quite large in the case of volume production-type CVD devices, not only does the aforementioned cleaning work require considerable time, but unless this cleaning work is performed properly, the problem of conversely increasing the amounts of particles and downfall ends up occurring. Thus, it is essential to reduce the levels of the aforementioned particles and downfall in order to improve the product yield of SiC epitaxial wafers.
Therefore, in the invention described in Patent Document 2 indicated below, adhesion of deposits (particles) deposited on the ceiling to wafers as a result of falling onto the wafers is proposed to be prevented by a cover plate by arranging a cover plate for covering the wafers between the wafers mounted on the susceptor and the ceiling (top plate) opposing the susceptor.
However, in this case, although deposits deposited on the ceiling can be prevented from falling onto the wafers, since the deposition of deposits on the ceiling cannot be prevented, bothersome cleaning work is required for removing the aforementioned deposits deposited on the ceiling.
On the other hand, a CVD device has also been proposed that inhibits particle generation by composing the ceiling with a large-sized SiC single crystal material and improving the adhesion of deposits to the surface of this ceiling (see Patent Document 3).
However, since a ceiling composed of a large-sized SiC single crystal material is susceptible to warping and cracking by high-temperature heating during film formation, it is difficult to stably use such a ceiling composed of a large-sized SiC single crystal material for a long period of time. In addition, large-sized SiC single crystal materials are also extremely expensive and SiC single crystal substrates in excess of 4 inches are difficult to acquire, thereby resulting in problems in terms of fabricating the CVD device as well.